1. Field of the Invention
The present invention relates to the field of semiconductor wafer processing and, more particularly, to controlling polishing temperature when performing chemical mechanical polishing on a linear planarization tool.
2. Background of the Related Art
The manufacture of an integrated circuit (IC) device requires the formation of various layers above a base semiconductor substrate, in order to form embedded structures over or in previous layers formed on the substrate. During the manufacturing process, certain portions of these layers need complete or partial removal to achieve the desired device structure. With diminishing feature size, such structures result in highly irregular surface topography causing manufacturing problems in the formation of thin film layers. To facilitate manufacturing processes, the rough surface topography has to be smoothened or planarized.
One of the methods for achieving planarization of the surface is chemical mechanical polishing (CMP). CMP is being extensively pursued to planarize a surface of a semiconductor wafer, such as a silicon wafer, at various stages of integrated circuit processing. CMP is also used in flattening optical surfaces, metrology samples, and various metal and semiconductor based substrates.
CMP is a technique in which a chemical slurry is used along with a polishing pad to polish away materials on a semiconductor wafer. The mechanical movement of the pad relative to the wafer, in combination with the chemical reaction of the slurry disposed between the wafer and the pad, provide the abrasive force with chemical erosion to planarize the exposed surface of the wafer (typically, a layer formed on the wafer). Typically, a downforce presses the wafer onto the pad to perform the CMP. In the most common method of performing CMP, a substrate is mounted on a polishing head and rotated against a polishing pad placed on a rotating table. The mechanical force for polishing is derived from the rotating table speed and the downward force on the head. The chemical slurry is constantly transferred under the polishing head. Rotation of the polishing head helps in the slurry delivery, as well as in averaging the polishing rates across the substrate surface.
Another technique for performing CMP to obtain a more effective polishing rate is using a linear planarization technology. Instead of a rotating pad, a moving belt is used to linearly move the pad across the wafer surface. The wafer is still rotated for averaging out the local variations, but the planarization uniformity is improved over CMP tools using rotating pads, partly due to the elimination of unequal radial velocities. In some instances, a fluid support (or platen) can be placed under the belt for use in adjusting the pad pressure being exerted on the wafer.
When a linear planarization tool is utilized, heat is generated by a variety of sources. At the pad surface where the pad engages the wafer, two factors contribute to heat generation. Heat is generated from the mechanical work, mostly the friction of the pad engaging the wafer. Heat is also generated from the exothermic chemical reaction of the slurry as CMP is performed. Transport of the heat energy away from the polishing tool is normally by natural convection to the ambient atmosphere or convection by the slurry as it is drained away from the pad. The remaining heat energy is stored in the tool, which will cause the tool temperature to rise.
The more critical temperature rise is noted in the polishing belt, as well as the pad material residing on the belt. Accordingly, a tool will experience a polish cycle to cycle global temperature rise as each subsequent wafer is polished on the tool. The temperature rise continues until an equilibrium temperature is reached. That is, when one wafer is processed immediately after another (without significant lag time between wafers), the belt temperature will rise, until some equilibrium temperature is reached. During this rise in temperature, it is appreciated that the polishing parameter or profile may vary from one wafer to the next as CMP is performed.
Once equilibrium temperature is reached, fairly consistent wafer polishing profile can be achieved, since the process temperature is stabilized. It should be noted that a significant number of wafers may need to be processed before this point is reached. FIG. 1 shows one experimental set of measurements. The graph of FIG. 1. shows temperature versus polishing time for a series of eight wafers polished one after the other. As can be seen from the intra-polish temperature profile of successive copper polish cycles overlaid on the graph, eight wafer polish cycles are required before the equilibrium temperature is reached. Since the first seven wafers were polished at less than the equilibrium operating temperature, the polishing profiles will vary due to the deviation in the process temperature of the wafer. The process temperature being the belt temperature (or at least very close to it). Therefore, some or all of these wafers may not be polished within the acceptable polishing tolerance, in which case the wafers may need rework or, worse, the wafers are scrapped. Scrapping 200 mm or 300 mm wafers is not very cost effective. At the least, repeatability of wafer polishing characteristic may not be achieved until the equilibrium temperature is reached.
Accordingly, it would be desirable to have a technique that provides for a more uniform cycle to cycle temperature repeatability when performing CMP.